The present invention relates to satellite-based positioning of location of terminals, and more particularly, to a rank-based positioning method suitable for a non-line-of-sight (NLOS) environment.
Satellite-based navigation systems such as the Global Positioning System (GPS) and the Global Navigation Satellite System (GNSS) are utilized to provide accurate position information to users (e.g. vehicle drivers). These satellite-based navigation systems require at least three satellites to determine the location of a user. When GPS signals from three satellites are received, a “two-dimensional” position (latitude and longitude) of the terminal may be determined. Further, when GPS signals are received from four or more satellites, a “three-dimensional” position (latitude, longitude, and altitude) may be determined.
Signals received in the open are not easily affected by the surrounding environment. Hence, measurable pseudo ranges are only affected by common noise sources, such as receiver thermal noise, ionospheric delay, tropospheric delay, and satellite clock bias. These noise sources are well-known and can be corrected through a prediction model and/or other reference information. Signals received in urban areas, however, are not only affected by the above noise sources, but are also affected by signal reflections in the surrounding environment.
For instance, in a downtown area, there are many high buildings which provide large reflection surfaces to reflect or block GNSS/GPS signals. These reflection surfaces may cause the receiver to receive signals reflected from many directions or block the receiver from receiving desired signal, which may lower the positioning accuracy. Unfortunately, this issue is not easy to solve, because the reflected signals usually change with location and time. The above scenario is the well-known multipath effect, which may generate extra measurement errors in pseudo ranges.
The multipath effect is the major issue in conventional positioning methods such as the Newton-Raphson method. Conventional positioning methods cannot provide accurate positioning results when encountering the multipath effect. Specifically, iterations of an objective function for calculating the receiver position and the clock bias cannot converge to correct position. Conventional methods require the existence of line-of-sight (LOS) signals to correctly determine the position of the receiver.
Therefore, there is a need for a pure software solution for positioning in a multipath environment.